A new study sheds light on how cigarette smoke accelerates eye aging.
Through a series of experiments, researchers say they have advanced understanding of how smoking damages the eye and contributes to the development of age-related macular degeneration.
Age-related macular degeneration is the leading worldwide cause of visual impairment and blindness among people age 50 and older.
It has long been known that people who smoke are four times more likely to develop age-related macular degeneration than non-smokers, but smoking’s role in eye disease development and progression has remained unclear.
In a report in the Proceedings of the National Academy of Sciences, the researchers describe how they compared how retinal pigmented epithelial cells—which protect and maintain light-sensing photoreceptors necessary for sight—changed in 3-month old and 12-month old mice after acute and chronic cigarette smoke exposure. These ages correspond to young adulthood and late middle age in humans.
“Smoking is often assumed to accelerate aging by releasing tissue-damaging molecules called free radicals,” says James T. Handa, principal investigator and chief of the retina division at the Wilmer Eye Institute at Johns Hopkins University.
The new study, Handa says, shows smoking also causes epigenetic changes—non-permanent shifts in gene expression that are not caused by changes in a cell’s DNA sequence—to retinal pigmented epithelial cells, or RPE cells, that have widespread effects on the eye and its ability to respond to environmental stress.
Using genetic sequencing techniques, the team studied RPE cells from mice three, six, and 10 days after they were injected with cigarette smoke condensate, and those exposed to cigarette smoke daily for four months. The researchers were able to identify dysfunctional RPE cells and understand how chromatin accessibility—the ability to physically access chromatin, a condensed structure of DNA, RNA, and proteins that control which genes are turned on or off—changed post-exposure. Such changes, if found, would indicate drastic shifts in a cell’s ability to adapt, function, and survive.
In both young and aged mice, acute exposure to injected cigarette smoke condensate caused the formation of dysfunctional RPE clusters with decreased expression of core RPE cell function genes, decreased chromatin accessibility, and decreased expression of “hallmarks of aging” genes—genes that prevent or regulate processes linked to aging. Such processes include genomic instability, shrinkage of the ends of chromosomes, and disruption of cells’ energy-producing mitochondria, among others.
Changes in chromatin arrangement caused by acute cigarette smoke stress limited the ability of young and aged mouse RPE cells to function, and replicated characteristics seen in humans with age-related macular degeneration, the researchers say.
Notably, the researchers say, they found that a separate, distinct subset of hallmarks of aging genes were expressed only in the dysfunctional cells of young mice treated with cigarette smoke condensate, but not their aged counterparts. Similar observations were made in young and aged mice that had been exposed to cigarette smoke daily for four months.
Conducting additional experiments with RPE cells donated by two people without age-related macular degeneration, or AMD, who did not smoke, one person without AMD who smoked, and one person with early AMD, the researchers identified 1,698 genes that either increased or decreased in expression and were shared between dysfunctional human and mouse RPE cells. Collectively, they suggested the shared hallmarks of aging genes may be relevant to AMD development and progression.
“Knowing environmental stress can interfere with the eye’s ability to produce the genes needed to stay healthy, we now want to narrow down which changes are temporary and which are permanent,” says Handa.
Building on their findings, Handa’s team plans on characterizing how age and continuous cigarette smoke exposure contributes to eye damage and comorbidities seen in patients with late-stage AMD.
The study was supported by the National Institutes of Health, the Research to Prevent Blindness Stein Innovation Award, and a BrightFocus Foundation macular degeneration research grant.
Handa is on the scientific advisory boards for Character Biosciences, Cirrus Pharmaceuticals, and Seeing Medicines.
Source: Johns Hopkins University
